1. Field of the Invention
The present invention relates to a technique of projecting an image upon applying trapezoid distortion correction.
2. Description of the Related Art
A projection apparatus such as a liquid crystal projector must often perform projection onto a projection plane such as a screen from, for example, above or below at a position at which it does not come up against the projection plane when, for example, its available installation space is limited or when it must be installed so as not to obstruct the observer's view. In such cases, the angle formed between the optical axis and the projection plane is different from a preset angle at which appropriate projection is possible, so distortion occurs in an image projected onto the projection plane. When, for example, a rectangular image is displayed, trapezoid distortion (keystone distortion) occurs.
A technique of detecting the distortion of a projected image, which has occurred due to factors associated with the method of projection onto the projection plane as in this case, and performing trapezoid distortion correction (keystone correction) for a projection image to make the projected image and the projection image similar to each other has been known. Some projection apparatuses even have a function of detecting the distortion of an image on the projection plane from the tilt angle of the projection apparatus and the shape of the projected image, and automatically performing trapezoid distortion correction.
In recent years, with, for example, the launch of digital television broadcasting, the resolution (the number of pixels) of video contents is increasing, so a demand for watching high-resolution video contents using a projection apparatus is naturally growing. To project high-resolution video contents by a projection apparatus, the clock rate for processing one pixel is raised with an increase in number of pixels to play back multi-pixel video contents because the time taken to process one frame of the video contents stays constant.
Unfortunately, when trapezoid distortion correction is performed while projecting high-resolution video contents by a projection apparatus, multiplication processing and memory access associated with trapezoid distortion correction are necessary, so the following problem may be posed. That is, when a high-rate clock is necessary to play back multi-pixel video contents, multiplication processing and memory access associated with trapezoid distortion correction often cannot be completed within one clock cycle, thus slowing down the frame rate.
Japanese Patent Laid-Open No. 2007-251723 discloses a technique of decreasing the number of bits of the color information of a video image with multi-pixel video contents, as mentioned above, thereby preventing a slowdown in frame rate.
However, because Japanese Patent Laid-Open No. 2007-251723 decreases the number of bits of the color information, image degradation may occur in an image to be projected. In trapezoid distortion correction processing accompanied by image enlargement processing, image degradation due to a decrease in number of bits of the color information often conspicuously appears.
Also, although a method of dividing multi-pixel video contents to perform trapezoid distortion correction processing in parallel by a plurality of image processing circuits to shorten the time taken to process the video contents is plausible, no concrete proposal has been presented until now.